1. Field of the Invention
The present invention relates to a method of making an austenitic stainless steel sheet having excellent surface evenness, uniformity and appearance after hot rolling.
2. Description of the Related Art
Austenitic stainless steels such as SUS304 have high heat resistance, corrosion resistance and workability, and are widely used for making various products by hot rolling, annealing, pickling, cold rolling, finishing annealing, and pickling.
The surface of a hot-rolled steel sheet is generally uneven because of the presence of surface scales formed during casting and hot-rolling the slab. When such a hot-rolled steel sheet is annealed in a general annealing atmosphere, that is, a combustive atmosphere, the steel sheet surface pattern has uneven glossiness or whiteness after pickling. This surface pattern damages the appearance of roofs and other panels made from the steel sheet.
In recent years, tandem rolling using a large roll has been applied to cold rolling of austenitic stainless steel, as well as plain carbon steel, in order to enhance productivity and to reduce production cost. Since such large roll does not effectively crush the surface defects or diminish intergranular penetration on the hot-rolled steel sheet, as compared with the use of conventional small rolls, the resulting cold-rolled steel sheet has remarkably uneven glossiness distribution.
Examples of surface defects include grooves formed by intergranular penetration, pit-type penetration in grains, and bite marks. In austenitic stainless steel, substrate barely dissolves during pickling. Hence the surface defects tend to remain on the hot-rolled sheet after pickling, as compared with ferritic stainless steel.
Various methods as follows have been proposed to minimize the foregoing surface defects in austenitic stainless steel.
An acid having strong dissolving ability can be used to completely dissolve groove-type corrosion and etched pits, as disclosed in Japanese Patent Laid-Open No. 60-248889. Since a large amount of scrap metal has been recently used as a source, the resulting austenitic stainless steel often contains rather large amounts of Cu, V and Mo. FIGS. 1A and 1B are graphs showing the solubility of SUS304 stainless steel sheets A, B, C containing these impurities, as shown in the following Table 9, in two acid mixtures of nitric acid and hydrofluoric acid (hereinafter referred to as nitric-hydrofluoric acid). The dissolving rate in pickling decreases with a increase of concentration of the impurities, probably due to surface passivation, a change in reaction potential, and the effect of nitride near the surface. Such a process requires a prolonged period to completely remove by dissolution the groove-type corrosion and etched pits from the surface of the steel sheet, resulting in a significant decrease of production speed and efficiency.
According to the present inventors' Japanese Patent Laid-Open No. 8-269549, mechanical descaling may be performed before annealing a hot-rolled steel sheet to minimize grooves of intergranular penetration for the purpose of improving the glossiness of the steel sheet. When scales are unevenly formed during hot rolling in this method, it is difficult to perform complete descaling and to remove unevenness from substrate texture. As a result, uneven glossiness on the steel surface still remains after such treatment, although the total glossiness is indeed improved.
Japanese Patent Laid-Open No. 60-177135 discloses a process including annealing for a short time in an inert or reductive gas or in vacuum and then rapidly cooling the steel sheet in order to suppress intergranular penetration of the hot-rolled steel sheet. This process, however, does not improve unevenness of the scales formed during hot rolling, and results in inevitable formation of a pattern on the surface of the steel sheet, even though suppressing formation of intergranular penetration during annealing.
Japanese Patent Laid-Open No. 6-10171 discloses a method for mechanically grinding a ferritic stainless steel sheet and then pickling it in nitric-hydrofluoric acid of a specified concentration. Austenitic stainless steel shows a quite different pickling mechanism as distinguished from that of ferritic stainless steel. That is, dissolution of austenitic stainless steel is significantly inactive when exposed to nitric-hydrofluoric acid because of the open circuit potential in the acid compared with that of the ferritic stainless steel. Thus, the surface defects on the austenitic stainless steel sheet cannot be removed using a pickling solution having an acid concentration that does not form so-called smuts, as disclosed in Japanese Patent Laid-Open No. 6-10171. The ferritic stainless steel significantly dissolves in sulfuric acid, whereas the austenitic stainless steel substantially does not do so. Accordingly, this method is not applicable to austenitic stainless steel.